Pyrolysis processes can be categorised as “fast pyrolysis” and “slow pyrolysis”, depending on the heating rate experienced by the carbon-containing feedstock. “Slow pyrolysis” processes typically involve long residence times and low heating rates (for example, less than 10° C. per second). Example slow pyrolysis processes include systems based on packed bed reactors, retorts and kilns. “Fast pyrolysis” processes involve the rapid decomposition of biomass under high heating rates (for example, greater than 100° C. per second) and favours the formation of products which are liquids at room temperature at the expense of solid carbon-containing products such as char. Typical fast pyrolysis processes involve feedstocks reduced to a small physical size which are moving in order to maximise heat and mass transfer rates, for example by the use of gas-fluidized beds, entrained flow reactors, rotary kilns or screw furnaces.
The pyrolysis of carbon-containing materials requires material to be heated in a non-oxidising, or “reducing” environment at moderate temperatures (eg from 200° C. up to 1000° C.). The process cannot be carried out in the open air or where free oxygen is present as the material to be pyrolysed will ignite and burn (oxidise) in the air at these processing temperatures, with the end result that the desirable products of the process are destroyed. Because of this, the process must be carried out in such a way that the presence of free oxygen is minimised. As heating is required to raise the temperature of the material and drive the pyrolysis process, a heat source (such as a combustion arrangement) is typically used. Because combustion requires oxygen, the combustion arrangement needs to be physically separated from the pyrolysis zone to prevent oxygen ingress into the pyrolysis zone.
In order to maintain a reducing environment, pyrolysis has in the past been undertaken by containing the material to be pyrolysed in a vessel or container from which air is excluded (usually by some form of air lock system or an arrangement of one-way valves) and which is then indirectly heated from the outside, ie with a solid barrier between heat source and pyrolysis zone.
Indirect heating may take the form of combustion of a separate fuel to provide the necessary heat input, or more often also involves the recovery and combustion of the pyrolysis product gases (volatiles) generated in the process by enabling them to be conveyed from the pyrolysis process into the combustion system and used as fuel.
This physical separation arrangement often involves one vessel inside another, or one pipe inside another, for example where the heating process occurs in the outer vessel or pipe, and the pyrolysis occurs inside the inner vessel or pipe. Electric elements or hot oil loops may also be used to provide heat into the material without exposure to oxidising conditions, however the operating cost of this approach limits its applicability. The presence of solid barriers also introduces limitations on scale-up, thermal efficiency and constrains options for the physical configuration of the process, which detrimentally affects process economics.
The physical arrangements necessary to minimise oxygen may also introduce additional requirements for feed preparation and product handling, in particular feedstock size reduction may become necessary to enable feed to be added and product withdrawn whilst maintaining the necessary reducing environment within the process.
The indirect heating methods commonly used in pyrolysis technologies add costs in construction (a separate sealed chamber being required for the pyrolysing charge, as well as the outer assembly containing the heating system). The indirect method also causes relatively poor transfer of heat from the heating system to the pyrolysing charge, as all heat must be conducted through the walls of the pyrolysis vessel, resulting in poor overall thermal efficiency.
Any discussion of documents, devices, acts or knowledge in this specification is included to explain the context of the invention. It should not be taken as an admission that any of the material formed part of the prior art base or the common general knowledge in the relevant art in Australia or any other country on or before the priority date of the claims herein.